Cap for a writing instrument



FIG. 1 is a front, right perspective view of a cap for a writing instrument showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a right side elevational view thereof, the left side being the same in appearance as the right side;

FIG. 4 is a rear elevational view thereof;

FIG. 5 is a top plan view thereof; and,

FIG. 6 is a bottom plan view thereof. 

The ornamental design for a cap for a writing instrument, as shown and described. 